Male albino rats will be used to investigate the discriminative stimulus attributes of morphine and of naloxone including relationships between each drug's stimulus attributes and the conditioned and unconditioned responses to the drug. The first series of experiments will train drug naive rats to make one response after receiving morphine and an alternative response after saline (M-S group). Another series of experiments will train morphine dependent rats to make one response after naloxone and the alternative response after saline (N-S group). Pharmacological inhibition of the discriminative response will be tested in the M-S animals by treating them with various doses of naloxone in combination with the training drug (morphine) and similarily the N-S animals will be treated with morphine in combination with the training drug (naloxone). Analgesic activity will be determined following most test sessions and some training sessions. The pA2 values derived from drug discrimination tests and analgesic tests will be compared. A third series of studies will associate a conditioned stimulus (CS) with the unconditioned drug effects of morphine or naloxone using Pavlovian classical conditioning, followed by tests for the ability of the CS to evoke the drug response in animals trained to make differential responses in the drug and saline condition. Discriminative response tests in both groups, combining the CS with drug or saline injection, will determine whether the conditioned effect is the direct drug response or a compensatory response. The effects of the CS on analgesia will be determined in both the morphine and naloxone trained animals.